Process of determining the suitability of steel for casehardening



. PROCESS OF DETERMINING THE SUITABILITY OF STEEL FOR CASE HA NNNNNN G.

IIllI llllll" I!!! UNITED STATES PATENT oFFicE.

HARRY W. MCQUAID. OF CANTON, OHIO, ASSIGNOR TO THE TIMKEN ROLLER BEARING COMPANY, OF CANTON, OHIO, A CORPORATION OF OHIO.

PROCESS OF DETERMINING THE SUITABILITY OF STEEL FOR CASEHARDENING.

To (ZZZ whom it may concern:

Be itknown that I, IIAHKY \V. McQUAn), a citizen of the United States, and a resident of the city of Canton, in the county of Stark and State of Ohio, have invented a new and useful Improvement in Processes of Determining the Suitability of Steel for Casehardening, of which the following is a specification.

My invention relates to a method of testing steel to determine whether it is suitable for use in the manufacture of devices that require to be hardened. Heretofore it has been commonly assumed that the ability of steel to react properly to the usual hardening treatment depended wholly upon its chemical composition and could be told in advance of the hardening operation by simple chemical analysis of the steel. I have discovered, however. that traces of impurities too minute to be detected by ordinary chemical analysis. may wholly unfit a steel for hardening; and it is the object of the present invention to devise a reliable method of determining whether steel of a. particular lot is suitable for hardening. It consists principally in carburizing a sample of such steel and microscopically examining the same as hereinafter specified.

In the accompanying drawing, which forms part of this specification, and wherein like reference letters refer to like parts wher ever they occur,

Fig. 1 is a diagrammatic view illustrating the characteristic appearance, under high magnification, of a polished and etched section through the hypereutectoid zone of a normal steel that hasbeen carburized and slowly cooled; and

Fig. 2 is a similar diagrammatic view illustrating the chararteristic appearence, under high magnification, of a polished and etched section through the hypereutectoid zone of an abnormal steel that has been carburized and slowly cooled.

In the accompanying drawing, the reference letter C indicates areas of cementite (Fe C) sometimes called iron carbide. The

reference letter P indicates pearlitic areas or areas of pearlite. The reference letter F indicates areas of ferrite.

In carrying out my process, I take a suitable sample of steel that is to be tested.

Specification of Letters Patent. Patented Aug. 15, 1922. Application filed August 1, 1921. Serial No. 488,885.

polished and etched surface of the metal is then examined under a microscope of sufficient magnification to clearly show the micro-structure of the section, say magnification of from fifty to two hundred diameters.

I have discovered that when two steels of apparently the same chemical composition, one of which is suitable for hardening (and which is herein referred to as the normal steel) and the other of which is not suitable for hardening (and which is hereinafter referred to as abnormal steel), are. case-carburized and slowly cooled and sections made through the hypereutectlc zone of their cases and suitably prepared for microscopic eXam-' ination, there is a. characteristic difference in their appearence. The etched normal sample shows large grains of pearlite separated by fine lines of cementite, so that the structure appears to be that of large grains of lamellar structure separated by fine lines of cementite. On the other hand, in the sample that is unfit for hardening, the grains are smaller in size and less uniform in appearance and the ridges of cementite are more numerous than in the normal sample. Evidence of the tendency of the pearlite areas to form free cementite on a background of ferrit'e is apparent. Presumably, the difference in the structure of the normal steel and of the abnormal steel is due to present in quantity dizing agents and which remain present in such minute quantity as to defy ordinary chemical analysis.

The compounds of other non-metallic elements, as well as those of oxygen, are segregated in minute particles that are distributed through the steel Without being dissolved therein, and, under microscopic examination, such particles are clearly difi'erentiated from the steel and present characteristic appearances by which they may be identified generically or specifically.

In determining microscopically whether or not the steel is normal or abnorn'ial it should be noted that the hypereutectoid zone is necessary to develop the breaking down of the pearlite and while the gradation zone of the abnormal steel can easily be distinguished from the gradation zone of normal steel after carburizing, the eutectoid zone shows little difference, the pearlite in the abnormal steel apparently having a slight tendency to become coarsely lamellar. The presence of cementite, however, apparently facilitates the breaking down of the pearlite and hence the higher the carbon concentration, i. e. the greater the amount of excess cementite, the more. complete will be the breaking down of the pearlite. Therefore, carburizing compounds which produce a very high carbon case will indicate more clearly whether or not the steel is normal or abnormal.

Annexed hereto are typical microphotcgraphs of the hypereutectoid zones of normal and abnormal steels after case hardening and slow cooling, the magnification in each case being two hundred diameters. In photograph No. 1, which shows the normal steel,- the pearlite is very finely lamellar and the cementite exists as well defined lines of fine but continuous formation at the boundiaries of the crystals. In photograph No. 2, which shows the abnormal steel carburized and cooled under the same conditions as the normal steel of photograph No. .1, the pearlite is broken down into the form of massive cementite and free ferrite, the cementite appearing as ridges on a white background of ferrite. While there will be differences in detail in photographs of nor mal steels, the appearance shown in photograph N0. 1 is characteristic; and likewise, the appearance typical or characteristic of an abnormal. steel is represented in photograph No. 2, although individual photographs will differ in detail.

For the purpose cl completing, studying and recording tests, microphoto raphs may be made of suitably polished and etched sections of carburizedsam les, By study and comparison of such p otomicrographs of normal and abnormal samples, one soon learns to recognize the appearances that characterize steel as fit or unfit for hardentamer/s? ing purposes; and photomicrographs of such samples may be preserved for purposes of record or study.

While the principal. object of the invention is to be able to determine from a carburized sample the suitability of a given lot of steel for properly reacting to, hardening treatment, it is noted that the carburized sample is not hardened, for the rapid quenching required for hardening produces a very difi'erent appearance from that of slow cooling. Likewise, as the characteristic appearance of abnormal steel is due to the presence of impurities therein and as such impurities affect the quality of the steel for other uses than hardening, the process above described is applicable to other purposes than simply determining the suitability of steel for hardening.

What I claim is: j 1. The process of determining the suitability of steel for hardening which consists in carburizing a sample thereof and microscopically examining a section of said sample through its carburized case.

2. The process of determining the suitability of steel for hardening which consists 1n case-carburizing and slowly cooling a sample thereof, preparing a section of the carburized case for microscopic examination, and microscopically examining such section. i

3. The process of determining the suitability of steel for hardening which consists in case-carburizing and slowly cooling a sample thereof, preparing a section of the carburized case for microscopic examination, and microscopically comparing such section with a similarly treated sample of steel of known standard qualities.

l. Theprocess of determining the suitability of steel for case-hardening which consists in case-carburizing and slowly cooling a sample thereof, and preparing and microscopically examining a section of said sample through the hypereutectoid zone of its carburizcd case.

5. The process of determining the suitably of steel for case-hardening which consists in case-carburizing and slowly cooling a sample thereof, making a microphotograph of a section of said sample through the h'ypereutectoid zone of the carburized case and comparing the same with a like photograph of a steel of known quality.

6; The process which consists in casecarburizing and slowly cooling a sample of steel, making a microphotograph of a section of said sample through the hypereutectoid zoneof the carburized case andcomparing the same with a like photograph of a steel of known quality.

7. The process of determining the suitability of steel for case-hardening which consists in case-carburizing and slowly cooling a sample of steel, polishing and etchof steel, polishing and etching a section of ing asection of said sample through the said sample through the hypereutectoid 10 hypereutectoid zone, making a microphotm. zone, making a microphotograph of such graph of such sectlon and comparing the section and comparing the same with a like same with a like photograph of a steel of photograph of a steel of known quality. known quality. Signed at Canton, Ohio, this 20th ay of 8. The process which consists in case- July, 1921.

carburizing and slowly cooling a sample HARRY W. Mo UAID. 

